Corrugated paperboard is used extensively for packaging goods and protecting them during storage and shipping. A corrugated paperboard manufacturing process typically starts with a single sheet of paper commonly referred to as a liner. To this liner is added a fluted medium, by the use of an adhesive, thereby forming a single-faced sheet of corrugated paperboard. Positive pressure may be used to press the fluted medium into intimate contact with the liner. Thus, there is little concern for crushing the flute tips or failing to achieve sufficient contact between the fluted medium and the liner so that glue is omitted from a flute line.
A second opposing liner must be adhesively applied to the flute tips of the single-faced corrugated sheet to form the double-faced corrugated paperboard. Typical corrugating machines apply adhesive to the flute tips by a rotating glue applicator roll and an opposing backing, or pressure, roll. The glue applicator roll turns at about the speed of the single-faced sheet, while the lower portion of the rotating roll picks up glue from a suitable reservoir. Glue is thus transferred from the rotating glue roll to the flute tips of the single-faced sheet. After the glue is applied to the flute tips, the second liner is brought into contact with the flute tips to form the double-faced corrugated paperboard as is typically used in corrugated packaging containers.
To ensure that the flute tips adequately contact the glue roll so that glue is applied to all of the flute tips of the single-faced sheet, pressure must be applied to the back, or liner side, of the single- o faced sheet. It is known in the art to use a rotating pressure roll positioned opposite the glue roll to ensure contact between the flute tips of the single- o faced sheet and the glue roll. For example, U.S. Pat. No. 4,316,755 to Flaum et al. discloses a typical rotating pressure roll positioned opposite the glue applicator roll.
Unfortunately, the opposing pressure roll of the prior art may frequently cause crushing of the flute tips as the single-faced sheet is pinched at the tangent point between the pressure roll and the glue roll. This is especially true as more recycled materials are used in the manufacturing of corrugated paperboard. The recycled materials typically have shorter fibers which make the flute tips formed of these shorter fibers more susceptible to crushing if the proper amount of pressure is not applied.
The U.S. Interstate Commerce Commission has recently removed certain regulatory restrictions requiring specific weights of paper to be used in corrugated paperboard manufacturing. Accordingly, many manufacturers are likely to vary the weight of the liners and the fluted medium in an effort to reduce cost while still producing a commercially viable product. Lessening the weight of the fluted medium, for example, is likely to further exacerbate the problem of crushing of the flute tips of the single-faced sheet caused by a rotating pressure roll.
It is also known in the art to provide an air bearing in place of the pressure roll to attempt to overcome crushing of the flute tips and other limitations associated with the rotating pressure roll. For example, U.S. Pat. No. 4,655,163 to Hokenson discloses an air bearing for pressing the single-faced sheet against the glue roll using an air pressure of about a half to a few psi above atmospheric pressure. The purpose for the air bearing disclosed in Hokenson is to cause the flute tips to lightly touch the glue roll to prevent "washboarding" from too much glue applied to the tips. The Hokenson air bearing has a plurality of air outlets supplied along its length and positioned opposite the rotating glue roll.
U.S. Pat. No. 4,617,076 to Jones, Jr. et al. similarly discloses an air bearing for providing contact of the flute tips with the rotating glue roll and having the stated purpose of preventing crushing of the flute tips. Similarly, U.S. Pat. No. 4,764,236 to Nikkel discloses an air bearing having a plurality of segmented shoes resiliently secured to a common manifold air source by springs. Nikkel discloses air bearing shoes of different widths on the outer and inner portions of the air bearing, thereby necessitating a larger inventory of spare parts for maintenance.
The air bearings typically have relatively complicated structures including air passageways, manifolds, and require a source of pressurized air. Small air passageways may readily clog in the presence of glue as used for corrugated paperboard manufacturing. In addition, the air bearing may spray glue throughout the machine if the air outlets of the air bearing are not completely covered by the single-faced sheet. Air bearings may also not provide sufficient protection against crushing of the flute tips, especially in light of the use of more recycled materials or in the case of a reduced weight fluted medium.
Thus, although the difficulty of pressing the single-faced sheet against an opposing glue roll without crushing the flute tips is recognized in the art, a simple, yet reliable, apparatus has not heretofore been developed for ensuring proper contact between the single-faced corrugated paperboard sheet and the rotating glue roll without crushing the flute tips of the sheet.